This invention is directed to the removal of ionic silver from solutions using halogen impregnated adsorbents. Said solutions are usually aqueous; however, non-aqueous solutions containing silver metal complexes are also within the scope of the instant invention. As used herein, the term adsorbent(s) refers to those porous organic and inorganic materials capable of being impregnated with halogen including: the crystallogenetic adsorbents (natural and synthetic zeolites), activated alumina, activated silica, fuller's earth (and other adsorbent clays, e.q., bentonite, hectorite), ion exchange resins and the like.
This invention is especially directed to the removal of ionic silver from photographic processing solutions such as, developers, fixers, bleach fixers and washwaters by contacting said solutions with a quantity of halogen impregnated non-carbon containing adsorbent.
The term "ionic silver" as used herein, is defined as including both the cationic form of silver, Ag.sup.+, the anionic silver thiosulfate complex, [Ag(S.sub.2 O.sub.3).sub.2 ].sup.-3, and other silver metal complexes.
The silver complex present in most photographic processing solutions is known to exist as a stable, soluble silver-thiosulfate complex. The red-ox potential for the reduction of this complex to elemental silver is quite low, E.degree.=0.01 V. Because of the low red-ox potential a strong reducing agent is necessary to carry out the reduction reaction.
Generally, the ionic silver present in photographic processing solutions is recovered by electrolytic methods. Generally, electrolytic recovery of ionic silver is useful only for ionic silver concentrations greater than 500 ppm. Typical electrolytic processes are described in U.S. Pat. Nos. 4,166,781; 4,111,766; and 4,026,784.
The disadvantages of the electrolytic method of silver removal from photographic solutions include; reducing the ionic silver concentration to below 500 ppm is difficult, capital expenditure for an electrolytic system is high, the system requires continuous monitoring and in addition to silver collecting at the cathode, the thiosulfate anion can be reduced to sulfide ion which immediately reacts with silver to form silver sulfide. This silver sulfide precipitate reduces the purity of the recovered silver.
Impregnated adsorbents have been used in numerous applications. Iodine has been recovered from a process stream via contact with alumina impregnated with oxides of copper and chromium (see U.S. Pat. No. 4,036,940 to McLane et al.). Inorganic adsorbents impregnated with acids and alkali metal salts have been employed in the purification of lactam compounds (see U.S. Pat. No. 3,960,846 to Potin et al.). Silica, impregnated with a transition metal oxide has been employed in devices involving electron bombardment (see U.S. Pat. No. 3,440,092 to Best et al.). Zeolites have been impregnated with alkali metals and used in the synthesis of acrylonitrile (see U.S. Pat. No. 3,527,784 to Smith).
Activated carbon has long been used as an adsorbent for removing metal ions from solutions. Carbon has been used in gold mining operations for the adsorption of Au.sup.+ (CN).sup.-, gold-cyanide since about 1880. The activated carbon is generally believed to act as a reducing agent, for example, adding an electron to the cationic metal (M.sup.+) to form the elemental metal (M.degree.). For example, the red-ox potential for the reduction of silver (Ag.sup.+) is high, E.degree.=0.8 V and a weak reducing agent is sufficient for carrying out the reaction. The elemental silver (Ag.degree.) is then loosely bound to the activated carbon surface.